1. Field of the Invention
The present invention relates to a semiconductor device provided with a circuit including a thin film transistor (hereinafter referred to as a TFT) using an oxide semiconductor film for a channel formation region and a method for manufacturing the semiconductor device. For example, the present invention relates to an electronic appliance having as a component an electro-optical device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element.
2. Description of the Related Art
As typically seen in a liquid crystal display device, a thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon. A thin film transistor manufactured using amorphous silicon has low field effect mobility, but can be formed over a large glass substrate. In contrast, a thin film transistor manufactured using crystalline silicon has high field effect mobility, but is not always suitable for being formed over a large glass substrate due to a crystallization step such as laser annealing.
In view of the foregoing, a technique by which a thin film transistor is formed using an oxide semiconductor and such a thin film transistor is applied to an electronic device or an optical device has attracted attention. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor for an oxide semiconductor film and such a thin film transistor is used as a switching element or the like of an image display device. Further, a technique by which an aperture ratio is increased with the use of light-transmitting electrodes as gate electrodes and source and drain electrodes has been considered (Patent Documents 3 and 4).